(1) Field of the Invention
The present invention relates to a system and method for controlling a speed of an electric motor in an extremely low speed range of the motor using a rotary pulse encoder as a motor speed detector.
(2) Description of the Background Art
Generally, in a previously proposed speed control system for an electric motor using a rotary pulse encoder having a relatively low resolution, each interval of adjacent pulses (or the period of each rising pulse) derived from the pulse encoder usually becomes longer than a speed control period (or sampling pulse interval derived from a reference clock of a microcomputer to derive the motor speed). When the motor rotates at an extremely low speed, accurate speed information cannot be obtained during the speed control period.
Therefore, the speed control system described above often becomes unstable in the extremely low speed rang 13 as described below with reference to FIGS. 1 (A) through 1 (D).
The pulse encoder linked across a rotation axle of the motor produces a pulse as shown in FIG. 1 (C) in the low speed range of the motor near zero.
In detail, a rotation speed n.sub.M with respect to a time t at a low speed range of the motor generally indicates a linear characteristic, as shown in FIG. 1 (A).
However, an angular position .theta. of the rotation axle of the motor with respect to time t generally indicates a curved characteristic as shown in FIG. 1 (B).
Hence, a pulse train derived from the pulse encoder has a pulse interval which becomes narrower as the time t has passed by as shown in FIG. 1 (C).
At a time when the pulse information of FIG. 1 (C) is changed, an average speed n.sub.M(j) for the pulse interval T.sub.p(j ),is derived as shown in FIG. 1 (D) according to the pulse interval T.sub.p(j) and a change rate of the pulse with respect to the pulse interval Tp(j). Therefore, if the pulse interval T.sub.p(j) is longer than the speed control period, the speed between each speed control period cannot be detected.
At this time, the average speed value using a previous value thereof n.sub.Mj-1 is used. Consequently, a deviation between the values of the average speed and real speed n.sub.M comes accordingly large as the motor speed becomes unstable in the lower speed range and in the speed control.
A Japanese Patent Application First Publication Heisei 2-307384 published on Dec. 20, 1990 exemplifies another previously proposed speed control system which has improved this unstable speed control characteristic.
In the disclosed Japanese Patent Application First Publication, the speed control system determines the rotation speed in the extremely low speed range using a load torque estimated value. However, since the means using the load torque estimated value is a type of a perfect order state observer, an adjustment of its gain (PI) is extremely difficult.
On the other hand, such an unstable control problem and a slow-responsive characteristic problem in the extremely low speed range of the motor rotation needs to be solved. Particularly, these problems need to be solve for applications of servomotor mechanisms and elevators demanding a high positioning accuracy.
Although a resolver or a highly precise rotary pulse output encoder has been used as the speed detector, many difficult problems such as adjustments and/or cost, problems in addition to the unstable control problem occur.
A Japanese Paper of T. IEE Japan, Vol. 107-D, No. 12, 1987 titled "Digital Servo Using Speed Estimation Observer" exemplified another previously proposed digital servo system using speed estimation observer (using the perfect order state observer) which is an expanded version of a load torque estimation observer.